For many optical applications a predetermined intensity distribution of a light beam is desirable. In particular in some cases even a very high uniformity of the light beam is desirable. An example where an intensity profile with very high uniformity is desirable is the so-called light induced crystallization processes where a thin and long laser beam is scanned over a thin non-crystalline thin film, i.e. an amorphous layer. These crystallization procedures are well known as excimer laser crystallization (ELC), sequential lateral solidification (SLS) or thin beam crystallization procedure (TDX). An overview of these different fabrication procedures is e.g. given by D. S. Knowles et al. “Thin Beam Crystallization Method: A New Laser Annealing Tool with Lower Cost and Higher Yield for LTPS Panels” in SID 05 Digest; Ji-Yong Park et al. “Thin Laser Beam Crystallization method for SOP and OLED application” in SID 05 Digest and in a brochure of the TCZ GmbH Company entitled “LCD Panel Manufacturing Moves to the next Level-Thin-Beam Directional 'Xtallization (TDX) Improves Yield, Quality and Throughput for Processing Poly-Silicon LCDs”.
In an optical system, consisting of a plurality of optical elements (as is the case for a laser annealing apparatus for carrying out such crystallization procedures), imperfections of the individual optical elements contribute to the total intensity and phase non-uniformity of the system.
Since the errors of individual components cannot be reduced arbitrarily, the total imperfection measured in the whole system is usually corrected at a certain individual element. In such sophisticated applications like optical and (extreme ultraviolet) EUV microlithography phase errors are locally corrected at one individual element by means of an ion beam figuring method (ion beam milling, ion beam etching) or by any kind of mechanical polishing, particularly computer-controlled polishing.